Preparation of high purity silicon by decomposition of silicon nitride powder having a graphite target buried therein



' Aug. 1963 L. M. HAGEN' ETAL PREPARATION OF HIGH PURITY SILICON BY DECOMPOSITION OF I SILICON NITRIDE POWDER HAVING A GRAPHITE TARGET BURIED THEREIN v 2 Sheets-Sheet 1' l Pure 3raphife Tane Filed Aug. 17, 1959 Quarlz Crucible Refine Comm'pcial Reacf Pure Silicon Heal Silicon Amide Te frachloride Wifh To Form Silicon Ni)- Silicon Yfrachloride By Disfillafion. I Pure Ammonia To ride. Form Silicon Amiole HeafSillcon Nilriole Crush Silicon Rehedl Purify Silicon By To Decompose IfInfo UnJi/l Subsfanlia/ly Zone Segrcgqfion. Silicon Nilrogem All Silicon Nifriolels Decompcscol.

' ATTORNEYS! 20, 3 L. M. HAGEN ETAL 3,10 57 PREPARATION OF HIGH PURITY SILICON BY DECOMPOSITION 0F i SILICON NITRIDE POWDER HAVING A GRAPHITE TARGET BURIED THEREIN Filed Aug. 17, 1959 Y 2 Sheets-Sheet 2 O a ue Quarfz H apshield 1//Work C ail IN VEN TORS f L A Wfifl/(f M A9445.

V 134 L TER 1 Asks Fr n 3,101,257 PREPARATION 'OF- HIGH PURITY SILICON BY DECOMPOSITION OF SILICON NITRIDE POW:

DER HAVING A GRAPHITE TARGET BURIED THEREIN Lawrence M, Hagen, Nashua, NH, and Walter F. Leverton, Weston, Mass, assignors, by mesne assignments, to theUnited States of America as represented by the :Secretaryof the Navy." t

- y; ;,Filed Aug. 17, -19;59 ,--Ser. No. 834,361

4 Claims. (Cl. 23-2235) This invention relates to the preparation of a high purity crystalline silicon of semiconductor grade, and particularly toanimproved method and -apparatus forobtaining such highfpurity silicon, employing the thermal decomposition of high purity siliconnit-ride.

An object of this invention is to provide an improved and relatively simple, practical, effective and inexpensive apparatus and method for producing such silicon of semiconductor grade.

Other objects andadvantages will be apparent from I the fiollowin'g description of an example of the invention and :the novel features Willbe particularly pointed out hereinafterlin connection withthe appended claims.

,Iill the accompanying drawings: FIG! 1 is. a schematic sectional elevational view of a crucible inductively heated and containing sulphur nitride which is thermally decomposed in accordance with this invention; i

FIG. 2 is a schematic diagram illustrating-a sequence of steps employed in accordance with this invention; and

FIG. 3I-i-s a schematic diagram illustrating in cross-section apparatus that may be employed for the multiple zone refining of the silicon in accordance with this invention.

Refer-ring first to FIG. 2, this invention comprises, as

the first step of the method, the refining of commercial silicon tetrachloride by distillation. This purified product so obtained, is then reacted with pure ammonia to produce a compound of silicon, nitrogen, and hydrogen, i.e., silicon amide, which represents the second step in the method. Silicon amide so obtained, is then heated to form silicon nitride, which is the third step in the method. The silicon nitride so obtained is then heated to cause thermal decomposition ol the silicon nitride into silicon and nitrogen, and since the nitrogen is a gas and escapes, it leaves only silicon. Usually :all of the silicon nitride is not decomposed in manner, and therefore the silicon product obtained by the thermal decomposition of the amide is crushed and reheated for further decomposition, with the crushing and reheating continued until substantially all of the silicon nitride has been decomposed in this manner. The resulting silicon so obtained, is then purified by zone segregation.

Using relatively impure commercial nitride'it has been found that the decomposition rate is rapid at temperatures that are easily obtained using RlF. induction heating. For example, several grams of com-merci-al silicon nitride can be decomposed in about 5 minutes at a temperature of approximately 1600 C.

The conversion of silicon nitride to silicon metal may be obtained, for example, by thermo decomposition in the tollowing manner:

Silicon nitride powder is placed in a quartz crucible with a spherical graphite target such as about /2" in diameter buried in the center of the nitride A below the surface. This graphite target is then heated .by induction to temperatures varying from about 1600 C. to 1700 C., which reduces the silicon nitride to a mixture which is about 50% silicon nitride and 50% silicon. This mixture of silicon and silicon nitride is then crushed and then reheated repeatedly for several times until substantially all 3,101,257 H Patented Aug. 20, 1963 This heating of the silicon nitride in the quartz crystal witht'he graphite target buried therein is illustrated FIG. 1, in which the quartz crucible .1 contains a quantity 2 of silicon nitride, and buried in this silicon nitride pow-' der is the pure graphite target 3. The induction heating coil 4 encircles the crucible and the energy created by the induction coil 4 is taken up by the graphite target and converted into heat which heats the surrounding powder and decomposes it into silicon metal andnitrogen.

i In order'to employ the crystalline metal so obtained as a semiconductor, it is advantageous to purify it and this purification-is preferably obtained by what is known as multi-zone purification or refining, and apparatus suitable for-that purposeis illustrated schematically in FIG. 3. The doping impurities, with the possible exception 'of boron, could, in principle, be removed rather efiiciently from silicon by a multiplezone segregation which has been used commercially for refining germanium. Early attempts to zone refine silicon failed because molten silicon wet the crucible which was usually formed of quartz and caused it to crack during the cooling cycle between molten zones.

According to the present invention, the cracking of the crucible has been greatly reduced or eliminated by maintaining the regions between the molten zones at relatively high temperatures, and apparatus such as illustrated in The crucible-5,'FIG. 3, is fabricated from clear quantz and i has a wall thickness from about 1 to 2 inm., and can,

for ex-ample, be about 45 cm. long, and of semicircular cross section approximately 2 cm. in diameter. This crucible 5 is elongated and placed in a horizontal position with its open face uppermost. It is then placed within and is supported by a-high-purity graphite tube 6, which is open at both ends, and is about cm. long, 3.5 cm. outside diameter, and about 5 mm. wall thickness. This graphite tube, which serves as an R.F. heater, is surrounded by a heat shield 7 formed of opaque quartz tube of about cm. long, 5 cm. outside diameter, and about 5 mm. wall thickness. The graphite heater 6 and 9, respectively, along the top for substantially their entire length so as to permit visibility of the silicon in the ingot '10 in the crucible. This assembly is placed in a clear quartz tube or envelope 11 which forms the complete enclosure for the heat shield, graphite tube, quartz crucible and the ingot of silicon, and it has an inlet 1% and an outlet 13 in an end wall thereof, so that an inert gas such as pure argon or helium may be circulated through the envelope to flush out the air and replace it with the inert gas. This flushing may be continuous or it may be discontinued after all the air has been removed from the envelope or enclosure.

An R.F. work coil 14 having a plurality of heating portions spaced apart along the length of the envelope provides an equal number of heating zones that by induction heat the graphite tube and through it the crucible and zones of the silicon ingot. This work coil is movable over the envelope and as it does so, there are three induction heating zones which move along the envelope and create three spaced apart molten zones in the silicon ingot, and between these molten zones, the ingot is maintained at a temperature greater than about 700 C. As the leading coil leaves the end of the ingot,

I the entire work coil is moved back exactly the distance closed :in FIG. 3 and described, herein for creating these moving-molten zones is new, particularly as applied to; silicon, and has greatly reduced the damage due to the cracking ofthe crucible as the crucible cools between the moltenuzones of the ingot. It is fou'nd that the crucible I under this manner of use does not, break duringthe usual; run, but atthe conclusion'of the process of refinea a merit, the crucible and ingot may crack during the cooling, but the quartzcrucible, attached to the ingot maybe removed 5 by soaking; in' hydrofiuoric acid. After typical runis with such apparatus it was found'that the resulting material was of the ,p-type, and thevariat-ion of resistivity alongthe length of the pulled crystals indi:

"cated that the principle doping impurity was =bor-on. Such apparatus'is trouble free in operation, and the progress of the spaced molten zones is visible through the envelope and throughthe slots 8 and 9, which are aligned with one another, above the crucible. It provides a very practical method and 'means for the removal of doping materials other than boron tromsilicon. Important fea-' tures of this apparatus and method ,are the-provision for maintenance of high temperatures throughout the ingot and crucible during the entire run, and the opaque quartz heat shield which permits operation at a reasonable power level. i

11. The method of preparing high purity silicon of semiconductor grade, which comprises heating by induction a body of silicon nitride powder having a graphite target buried in the interior thereof below the surface,

' to a temperaturebetween about 1600 Qan'd about 1700" C., then crushin gthe product andsuhject-ing the crushed product to the same heating, repeatedly until substantially all -of the silicon nitride has been decomposed into silicon.

2, The method according to claim 1, and then purifying the silicon so obtained by zone segregation in the presence of an inert gas.

3.1 The method of preparing a high purity "silicon of tion products, until substantiallyall of the siliconnitride has been converted into siliconM/herein the thermal'de composition is obtained by" induction heatingfof. the powder with atarget buried therein below the surface,

5 at a temperature between about'1600," C

and 1700 C., said target being ofpure graphite.

, .In the manufiacture of a silicon semiconductor, the method according to claim 1, and then zone segregating the product by induction heating'such product in :an

.20 inert atmosphere, in a tnansverse zone and progressively moving said zone from one end of the product toithe other. p p 7 References Cited'in the file of this patent 2 v UNITED STATES PATENTS 2,739,088" Pfann' Mar. 20,1956} 2,750,268 Erasmus et a1. June 12 1956 2,773,745 Butler et a1. a Dec; 11, 1956 2,805,133 Olson "Sept. 3, 1957 2,897,329 Matare July 28, 1959 2,904,404 Ellis Sept. 15, 1959' 2,905,798 Freutel Sept. '22, 1959 4 2,926,075 .Pfann "Feb. 23,1960

OTHER REFERENCES 4 J. Amer. Chem. Soc, 1930, vol. 52, pages 4 8-52;

semiconductor grade, whichcomprises subjecting a quan tity'ot silicon nitride powder alternately. to thermal de- 5 10 composition treatments andcrushings of the decomposi 

1. THE METHOD OF PREPARING HIGH PURITY SILICON OF SEMICONDUCTOR GRADE, WHICH COMPRISES HEATING BY INDUCTION A BODY OF SILICON NITRIDE POWDER HAVING A GRAPHITE TARGET BURIED IN THE INTERIOR THEREOF BELOW THE SURFACE TO A TEMPERATURE BETWEEN ABOUT 1600*C. AND ABOUT 1700* C., THEN CRUSHING THE PRODUCT AND SUBJECTING THE CRUSHED PRODUCT TO THE SAME HEATING, REPEATEDLY UNTIL SUBSTANTIALLY ALL OF THE SILICON NITRIDE HAS BEEN DECOMPOSED INTO SILICON. 